Groily,
I suppose you know this, but just in case, the original crankcase holes for the studs were 1/4-20 BSW and the studs took 1/4-26 CEI (or BSC) nuts. I know this contradicts what you said regarding BSF, but, over here, British only lists the 02-0051 as a CEI nut and British Fasteners says "Most, but by no means all, fasteners on post-War BSA's (through the late '60's, when it got more complicated) were CEI." So, maybe thats correct. Anyway, the thing I don't understand (really, one of a plethora of things I don't understand) is why you object to allen screws. Furthermore, you could get hex-head cap screw as well. With studs, you're always fussing with them staying in when you're taking off the nuts.
My sump plate is held by three 6mm bolts and one 1/4-20 BSW. The 6mm helicoils I did myself, the latter has been in since the '70s. I suppose I will be be barbequed for the mix and match of Whitworth and metric, but, alas!
Richard